Currently in the art, two methods are employed in a mixing machine which employs variable speed pulleys to drive the mixer's mixing attachment. In a first method, the fixed center distance method, the distance between the centers of the pulleys is fixed. The fixed center distance method employs two variable speed pulleys linked by one drive belt which are used to transmit power and change the speed of the mixer. In a second method, the adjustable center distance method, the distance between the centers of one or two sets of pulleys can be adjusted. To provide a wide speed range, two sets of pulleys are required. This method employs two variable speed pulleys, two fixed diameter pulleys and two drive belts which are arranged so that they transmit power and change the speed of the mixer.
The drive train of a conventional adjustable center distance drive mixing machine with a wide range speed change is known in the art as an "extended" drive train. This type of drive train is driven by a motor which is typically positioned at or near the base of the mixing machine. This extended drive train comprises a first fixed diameter pulley which is driven by the motor. The fixed diameter pulley is linked by a drive belt to a first variable speed pulley. This first variable speed pulley is mounted on a moveable axle on which a second variable speed pulley is also mounted. The second variable speed pulley is linked via a second drive belt to a second fixed diameter pulley which is linked to the mixing head drive mechanism.
This conventional drive train has several drawbacks. To provide these mixers with a manageable size, the length of this type of drive train is limited by the height of the mixer. In these mixers, the motor which drives the drive train is mounted in the base of the mixer and the attachment drive is located in the mixer head. Typically, the distance between the centers of the first fixed diameter pulley and the first variable speed pulley and the distance between the centers of the second variable speed pulley and the second fixed diameter pulley equal approximately half of the center distance between the first fixed diameter pulley and the second fixed diameter pulley. As one can imagine, to design a mixing machine of manageable size, the distance between the first fixed diameter pulley and the second fixed diameter pulley is limited by the acceptable height of the mixing machine. The arrangement of the extended drive train requires the mixer to have a base foot print equal in length at least to the length of the motor plus the length of the motor drive shaft on which the first fixed diameter pulley is mounted.
It is known in the art that as the size of a pulley's pitch diameter decreases, the drive belt operating on that pulley will have a shorter fatigue life than if the same belt were placed on a pulley having a larger pitch diameter. The reduced fatigue life of the belt caused by the smaller diameter pulley results from the sharp degree of the bend such a pulley places in the belt. In other words, the sharper the bend placed in the belt by the pulley, the shorter the belt life will be. In conventional extended drive train mixers, the pulley on the motor drive shaft has a small pitch diameter because the size of that pulley is limited by the speed reduction ratio of the desired drive train of the mixer.
An additional problem that has plagued large volume mixing machines is known as "shifter creep". In these large volume mixing machines, a shift lever mounted on the outside of the machine is used to adjust the speed of the mixer's attachment drive. The shift lever is linked to the drive train of the attachment drive by a linkage. Movement of the shift lever in a first direction causes the speed of the attachment drive to increase while movement in a second direction causes the speed of the attachment drive to decrease. Under the forces generated by the rotation of the beaters and the vibration of the mixer, the shift lever has a tendency to vibrate out of position and change the speed of the attachment drive. As the mixing machine vibrates, the vibrations cause the shift lever to move in a direction which causes the speed of the attachment drive to increase or decrease. The undesired movement of the shift lever often results in the mixing ingredients being thrown out of the mixing bowl, overload of the mixer, increased mixing times and incomplete mixing of the ingredients in the mixing bowl.
The present invention provides two improvements over the prior art. First, the mixer provides a novel drive train for the attachment drive which provides greater power output to the mixing head and longer belt life for the belts used in the drive train than in conventional mixers. Second, the mixer provides a novel mechanism to maintain the shift lever of the mixing machine at the selected speed position and, subsequently, to provide for constant mixing at that speed until the mixer is either shut off or the speed is changed by the operator.